I was in a similar position when I bought my M3. The dealership where I purchased my M3 low-balled me with a trade offer for my 2008 550i. I went to several other dealerships and got trade-in offers for $4-5k more. Ultimately, I sold it myself for $12k more than the original trade-in offer.

It took me about 6 weeks to close a private party sale. Yes, I had to deal with a few phone calls, e-mails and negotiations but it was worth it in the end.